In a mobile communication using the CDMA system, a plurality of mobile stations shares the same frequency band, and each mobile station spreads transmission data by a spread code and transmits the data toward a base station. The individual mobile stations are assigned different spread codes having orthogonality with respect to each other, The base station is able to extract reception data from a received signal transmitted from a particular mobile station by performing inverse spread using the same spread code used for spread by the mobile station.
However, as the number of multiplex stations of mobile stations increases, the communication quality is deteriorated by the interference between different stations based on mutual correlation characteristics of the spread code. As the number of multiplex stations of the mobile stations further increases, connection of a call becomes difficult. On the contrary, it can be said that restraining the interference between the different stations permits improved efficiency of frequency utilization, improved communication quality, and an increased line capacity to be achieved. Hence, in the mobile communication using CDMA system, the restraint of the interference between the different stations is an important problem.
As a method for restraining the interference between the different stations in a direct spread CDMA system mobile communication, a method using an adaptive array antenna has been extensively studied, and many reports have been made.
The adaptive array antenna uses an array antenna constructed by a plurality of antennas. In such an adaptive array antenna, the directivity can be imparted to the entire array antenna by weighting the amplitude and phase of the reception output of each antenna and by synthesizing the weighted outputs. Furthermore, by changing the weighting, the directivity of the array antenna can be changed. The interference between the different stations can be effectively restrained by applying the reception using the adaptive array antenna (hereinafter referred to as “the array antenna reception”) to the mobile communication and by implementing a good directivity pattern by changing the weighting on the basis of a predetermined algorithm.
Generally, however, radio receiving units have different amplitude fluctuations and phase shifts due to variations in the elements making up amplifiers. In addition, the amplitude fluctuations and phase shifts change with time. Since the variations in these amplitude fluctuation and phase shift lead to a difference between a directivity pattern expected to be obtained by the control based on an algorithm and an actually obtained directivity pattern, the variations are barriers in implementing effective restraint of the interference between the different stations.
In order to correct the aforementioned variations, an operation called “calibration” has been performed, in which a known signal is received by each radio receiving unit, the amplitude fluctuations and phase shift of each are measured, and the measurement results are fed back in weighting and synthesizing.
The following will briefly explain a few conventional calibration methods.
(First Conventional Example)
As it will be explained in detail hereinafter, according to a first conventional calibration method, the spread code that is different from the spread code allocated to a mobile station for telephone calls is used to spread a known calibration signal, and the calibration signal is superimposed on a reception output of the antenna.
(Second Conventional Example)
As it will be explained in detail hereinafter, according to a second conventional calibration method, a reception output of an antenna is cut off, and only a calibration signal is received.
(Third Conventional Example)
According to a third conventional calibration method, a known signal is multiplexed in a time-division manner into a signal transmitted by a mobile station. A radio receiving apparatus in the third conventional calibration method separates the calibration signal that has been multiplexed in the time-division manner from a received signal and uses it for calculating a correction amount.
The radio receiving apparatuses in the conventional examples described above pose the following problems.
In the first conventional example, the calibration signal becomes a noise relative to a call signal transmitted by a mobile station, resulting in deteriorated speech quality. On the other hand, a call signal becomes a noise relative to the calibration signal, resulting in deteriorated accuracy of calibration.
In the second conventional example, a call of a mobile station is cut off, so that the call service is interrupted during calibration.
In the third conventional example, the communication capacity that a mobile station can use for calls is decreased by the calibration signal multiplexed in the time-division manner.
Accordingly, an object of the present invention is to provide a radio receiving apparatus that uses an adaptive array antenna and performs calibration without deteriorating speech quality.
Another object of the present invention is to provide a calibration method for the foregoing radio apparatus.